CPFSK receiver architecture

I'm evaluating receiver structures for the ETSI DMR spec, but not having
much luck finding any work in this area.  They use continuous phase
modulation, so I anticipate I'll have to use a viterbi demodulator to get
it done.  Unfortunately they choose h=.27, so I have at least 200 states,
times whatever number of symbol cycles their pulse shape is over (not
clear on this, three maybe?).  So I'm looking at 600*4 = 2400 matched
filters I have to evaluate per symbol to get branch metrics for the
viterbi, woof.  

If anyone can point me to revelant work I'd appreciate it, I'm really
looking for a receiver that's simpler than the optimum one while still
getting close in performance (some loss is unavoidable clearly).  If
anyone knows of one specifically for DMR that'd be great, as it's a little
weird.  

Lastly, perhaps someone can enlighten me on the difference between
"coherent" and "incoherent" receivers for CPM.  To me, coherent usually
means I know the carrier phase.  But, in CPFSK, if I knew the phase,
wouldn't I know everything I need to know?
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gct <24505@DSPRelated> wrote:

>I'm evaluating receiver structures for the ETSI DMR spec, but not having
>much luck finding any work in this area.  They use continuous phase
>modulation, so I anticipate I'll have to use a viterbi demodulator to get
>it done.  Unfortunately they choose h=.27, so I have at least 200 states,
>times whatever number of symbol cycles their pulse shape is over (not
>clear on this, three maybe?).  So I'm looking at 600*4 = 2400 matched
>filters I have to evaluate per symbol to get branch metrics for the
>viterbi, woof.  

>If anyone can point me to revelant work I'd appreciate it

You probably do not need a maximum likelyhood demodulator.  You
probably just need a standard discriminator type FM demodulator.
You need to look at your specs, perhaps do some simulations and determine 
whether you need something fancier, but I am guessing not, since such 
standards are aimed at cheaper implementations.

h = 0.27 does not even sound like it is supposed to be coherent.

Steve

On 04.07.2017 7:29, Steve Pope wrote:
> gct <24505@DSPRelated> wrote:
>
>> I'm evaluating receiver structures for the ETSI DMR spec, but not having
>> much luck finding any work in this area.  They use continuous phase
>> modulation, so I anticipate I'll have to use a viterbi demodulator to get
>> it done.  Unfortunately they choose h=.27, so I have at least 200 states,
>> times whatever number of symbol cycles their pulse shape is over (not
>> clear on this, three maybe?).  So I'm looking at 600*4 = 2400 matched
>> filters I have to evaluate per symbol to get branch metrics for the
>> viterbi, woof.
>
>> If anyone can point me to revelant work I'd appreciate it
>
> You probably do not need a maximum likelyhood demodulator.  You
> probably just need a standard discriminator type FM demodulator.
> You need to look at your specs, perhaps do some simulations and determine
> whether you need something fancier, but I am guessing not, since such
> standards are aimed at cheaper implementations.
>
> h = 0.27 does not even sound like it is supposed to be coherent.
>
> Steve
>

Gct, as for the relevant work, you've surely looked up the first link in 
Google Scholar for "etsi dmr cpfsk", haven't you?

http://ieeexplore.ieee.org/abstract/document/5304247/

Gene

>gct <24505@DSPRelated> wrote:
>
>>I'm evaluating receiver structures for the ETSI DMR spec, but not
having
>>much luck finding any work in this area.  They use continuous phase
>>modulation, so I anticipate I'll have to use a viterbi demodulator to
get
>>it done.  Unfortunately they choose h=.27, so I have at least 200
states,
>>times whatever number of symbol cycles their pulse shape is over (not
>>clear on this, three maybe?).  So I'm looking at 600*4 = 2400 matched
>>filters I have to evaluate per symbol to get branch metrics for the
>>viterbi, woof.  
>
>>If anyone can point me to revelant work I'd appreciate it
>
>You probably do not need a maximum likelyhood demodulator.  You
>probably just need a standard discriminator type FM demodulator.
>You need to look at your specs, perhaps do some simulations and determine

>whether you need something fancier, but I am guessing not, since such 
>standards are aimed at cheaper implementations.
>
>h = 0.27 does not even sound like it is supposed to be coherent.
>
>Steve

Yeah the modulation index throws me too, it'd be so easy to use .25 if
they wanted to make things easy.  So you'd probably just use an FM demod,
apply RRC filter, then try to discriminate the frequency in the middle of
the symbol?  Any idea how that compares to the "ideal" receiver?
---------------------------------------
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>On 04.07.2017 7:29, Steve Pope wrote:
>> gct <24505@DSPRelated> wrote:
>>
>>> I'm evaluating receiver structures for the ETSI DMR spec, but not
having
>>> much luck finding any work in this area.  They use continuous phase
>>> modulation, so I anticipate I'll have to use a viterbi demodulator to
>get
>>> it done.  Unfortunately they choose h=.27, so I have at least 200
>states,
>>> times whatever number of symbol cycles their pulse shape is over (not
>>> clear on this, three maybe?).  So I'm looking at 600*4 = 2400 matched
>>> filters I have to evaluate per symbol to get branch metrics for the
>>> viterbi, woof.
>>
>>> If anyone can point me to revelant work I'd appreciate it
>>
>> You probably do not need a maximum likelyhood demodulator.  You
>> probably just need a standard discriminator type FM demodulator.
>> You need to look at your specs, perhaps do some simulations and
determine
>> whether you need something fancier, but I am guessing not, since such
>> standards are aimed at cheaper implementations.
>>
>> h = 0.27 does not even sound like it is supposed to be coherent.
>>
>> Steve
>>
>
>Gct, as for the relevant work, you've surely looked up the first link in

>Google Scholar for "etsi dmr cpfsk", haven't you?
>
>http://ieeexplore.ieee.org/abstract/document/5304247/
>
>Gene

I surely did look it up, but they're doing a non-coherent detection, which
I was trying to avoid for performance reasons.  You can see they're at
least 1.7dB above the lower bound.
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On 04.07.2017 22:46, gct wrote:

(snip)

>
> I surely did look it up, but they're doing a non-coherent detection, which
> I was trying to avoid for performance reasons.  You can see they're at
> least 1.7dB above the lower bound.
> ---------------------------------------
> Posted through http://www.DSPRelated.com
>

Apologies if I sounded too harsh; I've read that article, too.

Just some thoughts (not from actual experience). Coherent detection 
doesn't have to mean Viterbi decoder. What about M-algorithm? Perhaps 
you could do better than non-coherent detection without too much complexity.

Best regards,
Gene

>On 04.07.2017 22:46, gct wrote:
>
>(snip)
>
>>
>> I surely did look it up, but they're doing a non-coherent detection,
>which
>> I was trying to avoid for performance reasons.  You can see they're at
>> least 1.7dB above the lower bound.
>> ---------------------------------------
>> Posted through http://www.DSPRelated.com
>>
>
>Apologies if I sounded too harsh; I've read that article, too.
>
>Just some thoughts (not from actual experience). Coherent detection 
>doesn't have to mean Viterbi decoder. What about M-algorithm? Perhaps 
>you could do better than non-coherent detection without too much
>complexity.
>
>Best regards,
>Gene

No worries, I haven't heard of the M-algorithm, do you have any good
references I could look at?
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gct <24505@DSPRelated> wrote:

> spp wrote,

>>You probably do not need a maximum likelyhood demodulator.  You
>>probably just need a standard discriminator type FM demodulator.
>>You need to look at your specs, perhaps do some simulations and determine

>>whether you need something fancier, but I am guessing not, since such 
>>standards are aimed at cheaper implementations.

>>h = 0.27 does not even sound like it is supposed to be coherent.

>Yeah the modulation index throws me too, it'd be so easy to use .25 if
>they wanted to make things easy.  So you'd probably just use an FM demod,
>apply RRC filter, then try to discriminate the frequency in the middle of
>the symbol?  Any idea how that compares to the "ideal" receiver?

(Typically no RRC filter in such a system.  Instead, use a DFE to 
subtract out the delay spread from the transmit filter and/or channel.)

In AWGN, usually between 1 and 2 dB worse.  It depends on the operating
point, which depends on the amount of coding etc.  If it is uncoded
and you need to get a low BER like 1e-8 then the differences can 
be profound.  But this is seldom the case in a real system.

In multipath .. a bit hard to say.  The degradation could be worse.

Good luck-
Steve

On 05.07.2017 2:53, gct wrote:
>> On 04.07.2017 22:46, gct wrote:
>>
>> (snip)
>>
>>>
>>> I surely did look it up, but they're doing a non-coherent detection,
>> which
>>> I was trying to avoid for performance reasons.  You can see they're at
>>> least 1.7dB above the lower bound.
>>> ---------------------------------------
>>> Posted through http://www.DSPRelated.com
>>>
>>
>> Apologies if I sounded too harsh; I've read that article, too.
>>
>> Just some thoughts (not from actual experience). Coherent detection
>> doesn't have to mean Viterbi decoder. What about M-algorithm? Perhaps
>> you could do better than non-coherent detection without too much
>> complexity.
>>
>> Best regards,
>> Gene
>
> No worries, I haven't heard of the M-algorithm, do you have any good
> references I could look at?
> ---------------------------------------
> Posted through http://www.DSPRelated.com
>

M algorithm is briefly reviewed in Todd Moon, "Error Correction Coding: 
Mathematical Methods and Algorithms".

Its description is actually so short that I could just quote it here. 
For fun's sake, here's what the text says (pp. 521-522):

-----------------------------------------

12.8.6 A Variation on the Viterbi Algorithm: the M Algorithm

For a trellis with a large number of states at each time instant, the 
Viterbi algorithm can be very complex. Furthermore, since there is only 
one correct path, most of the computations are expended in propagating 
paths that are not be used, but must be maintained to ensure the 
optimality of the decoding procedure. The M algorithm (see, e.g., [270]) 
is a suboptimal, breadth-first decoding algorithm whose complexity is 
parametric, allowing for more complexity in the decoding algorithm while 
decoding generally closer to the optimum.

A list of M paths is maintained. At each time step, these M paths are 
extended to M*2^k paths (where k is the number of input bits), and the 
path metric along each of these paths is computed just as for the 
Viterbi algorithm. The path metrics are sorted, then the best M paths 
are retained in preparation for the next step. At the end of the 
decoding cycle, the path with the best metric is used as the decoded 
value. While the underlying graphical structure for the Viterbi 
algorithm is a trellis, in which paths merge together, the underlying 
graphical structure for the M algorithm is a tree: the merging of paths 
is not explicitly represented, but better paths are retained by virtue 
of the sorting operation. The M-algorithm is thus a cross between the 
stack algorithm (but using all paths of the same length) and the Viterbi 
algorithm. If M is equal to the number of states, the M algorithm is 
nearly equivalent to the Viterbi algorithm. However, it is possible for 
M to be significantly less than the number of states with only modest 
loss of performance.

Another variation is the T-algorithm. It starts just like the M 
algorithm. However, instead of retaining only the best M paths, in the T 
algorithm all paths which are within a threshold T of the best path at 
that stage are retained.

-----------------------------------------

[270]: G. Pottie and D. Taylor, "A Comparison of Reduced Complexity 
Decoding Algorithms for Trellis Codes," IEEE Journal on Selected Areas 
in Communications, vol. 7, no. 9, pp. 1369-1380, 1989.

-----------------------------------------

If you follow the link, it would say something along the lines that 
M-algorithm is global, while Viterbi algorithm is local. Which makes 
some sense to me.

Best regards,
Gene

>gct <24505@DSPRelated> wrote:
>
>> spp wrote,
>
>>>You probably do not need a maximum likelyhood demodulator.  You
>>>probably just need a standard discriminator type FM demodulator.
>>>You need to look at your specs, perhaps do some simulations and
determine
>
>>>whether you need something fancier, but I am guessing not, since such 
>>>standards are aimed at cheaper implementations.
>
>>>h = 0.27 does not even sound like it is supposed to be coherent.
>
>>Yeah the modulation index throws me too, it'd be so easy to use .25 if
>>they wanted to make things easy.  So you'd probably just use an FM
demod,
>>apply RRC filter, then try to discriminate the frequency in the middle
of
>>the symbol?  Any idea how that compares to the "ideal" receiver?
>
>(Typically no RRC filter in such a system.  Instead, use a DFE to 
>subtract out the delay spread from the transmit filter and/or channel.)
>
>In AWGN, usually between 1 and 2 dB worse.  It depends on the operating
>point, which depends on the amount of coding etc.  If it is uncoded
>and you need to get a low BER like 1e-8 then the differences can 
>be profound.  But this is seldom the case in a real system.
>
>In multipath .. a bit hard to say.  The degradation could be worse.
>
>Good luck-
>Steve

Curious why no RRC filter, wouldn't it make sense to apply it then use the
DFE to remove any extra ISI?  Thanks for the help!
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